THE WETTABILITY OF MINERAL SURFACES CONTAINING ADSORBED ASPHALTENES

Studies on the adsorption of asphaltenes, extracted from shale oil and dissolved in toluene on various minerals, were carried out. Langmuir-type isotherms were obtained showing a maximum adsorption density of 1-2 mg m-2 for most minerals. The adsorption process produced marked changes in the wettability of the solid. The contact angles measured at the mineral-water-toluene solution interface for asphaltene-covered surfaces at neutral pH were 102-degrees for quartz, 89-degrees for calcite, 72-degrees for magnesite and 110-degrees for mica, whereas the corresponding values for the bare minerals were 32 +/- 2-degrees. Confirmatory results were obtained using immersion tests for powdered mineral samples. The contact angle depends on the aqueous phase, going through a maximum between pH 3 and pH 8; similar results were obtained for the interfacial tension between water and a solution of asphaltene in toluene. For low asphaltene concentrations the contact angles reverted to lower values on exposure of the mineral with adsorbed asphaltenes to the pure solvent. However, this inversion was not possible for higher asphaltene concentrations. Similar behavior was observed in previous work for the displacement of asphaltenes from the mineral-solution interface by non-ionic surfactants. These results were ascribed to lateral interactions that make the adsorption process irreversible for high surface coverage.